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Subject: [Amps] ALPHA 77 SX UPDATE
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Wed, 21 Aug 2002 14:05:36 +0100
2 wrote:
>Another consideration is that 2, 8877s have 0.2pF of feedback-C.  Since 
>the 8877 is capable of oscillating) well into the uhf range, this 
>amount of feedback-C is nothing to sneeze at.

Two tubes also have double the input capacitance and half the input 
impedance, so the feedback situation remains the same.

> [intermittent vhf/uhf parasites have a reputation for causing 
>intermittent hv-to-gnd arcs, so lower vhf-Q suppressors might not be a 
>bad idea]
If the grid is nailed down firmly - VHF-style, using multiple contact 
fingers on the grid ring - then it's questionable whether any VHF 
parasitic suppressors are needed in a single-8877 amp for HF. After all, 
the same tube is quite stable in VHF amps, where the input and output 
are both *deliberately* tuned to the same frequency! The same of course 
applies to the 3CX800 and similar ceramic tubes with a full grid ring.

However, with two tubes in parallel there is also the possibility of 
push-pull VHF resonances, so parasitic suppressors would probably be a 
good idea to kill this mode.

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'

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